$unionWith combines pipeline output from another collection into the current output stream. Unlike $lookup, it does not attach related documents to each source document. It does not automatically deduplicate equivalent records.

Normalize different source shapes with $project before combining them. If logical duplicates must be removed, define what makes records equivalent and add an explicit deduplication stage.

db.orders.aggregate([
  { $project: { orderId: "$_id", customerId: 1, status: 1, createdAt: 1, source: { $literal: "current" } } },
  {
    $unionWith: {
      coll: "archivedOrders",
      pipeline: [
        { $project: { orderId: "$_id", customerId: 1, status: 1, createdAt: 1, source: { $literal: "archive" } } }
      ]
    }
  },
  { $sort: { createdAt: -1, orderId: -1 } }
]);